To heat or cool an object, there are heat pump apparatuses that convey heat by using refrigerants. An example of a heat pump apparatus is an air-conditioning apparatus, or the like. A typical heat-pump air-conditioning apparatus includes a compressor, a four-way valve, an outdoor-side heat exchanger, an expansion valve, and so forth in an outdoor unit. In addition, the heat-pump air-conditioning apparatus includes an indoor heat exchanger in an indoor unit. These component devices are connected via refrigerant pipes to form a refrigerant circuit.
In such an air-conditioning apparatus, an inverter device controls a driving frequency, that is, a rotation speed (driving frequency) of the compressor, in accordance with a frequency instruction from a controller (control device). At this time, signals from sensors (detection devices) that are provided on a suction side of the compressor and that detect a compressor inlet temperature, a saturation temperature, and the like are input to the controller. Then, the controller serving as a control device determines a degree of superheat in the refrigerant circuit from detected values of the sensors contained in the signals, and controls an opening degree of the expansion valve so that the degree of superheat becomes constant (for example, see Patent Literature 1).
The controller performs the control described above, a refrigerant in a two phase region in which a heat transfer rate of the refrigerant is high is thereby used effectively, and thus the air-conditioning apparatus can be operated efficiently.